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Full text of "Text Book Of Mechanical Engineering"

Stress-strain Diagrams.

385

occurs.    This apparatus has been applied  in  Professor  Hele-
Shaw's machine.

Stress-strain Diagrams, as obtained principally by the
previous apparatus, will now be shewn (see Figs. 345 and 346).
The largest number of experiments have been made in tension
and our list of compression and shear diagrams is but meagre
In every case the authority has been cited, and where possible
the unit stress and length of specimens given.

Deductions.—Mild steel and good wrought iron have long
plastic extensions and considerable contraction at rupture (see
c, F, G, L), Stronger steels are less ductile, as at B and D,
while steel castings, A, are very short, though the strength may be
higher than shewn. Cast iron, Q, has really no elastic stage,
though Hodgkinson fixed an apparent limit, but brass, o, is
better off, and is much more plastic. N is a very fine diagram
for aluminium bronze, shewing great ductility and high elastic
limit. Torsional and transverse diagrams (s and R) are not
essentially different from tension in character, but compression
diagrams take quite a different form, v being a typical example,
the plastic portion tending always to curve in an opposite
direction to that of tension. T is an experiment on long pillars
held loosely in sockets to prevent bending; and diagrams Q, x, u,
and v have all been plotted.

Raising the Elastic Limit.—If the load be carried a little
beyond the primitive elastic limit and allowed to remain, say, for
24 hours, then removed, the bar will strain slightly -y but on re-
stressing, a new elastic limit will be found at a little higher load
than that just removed. Repeating the experiment beyond the
second limit, a third limit may be found, and so on until the bar
breaks. All this is beautifully given by diagram M, and also by
diagram s, one plastic curve bounding all the limits, and it is
clearly shewn why English engineers consider the breaking load
the only reliable test of a material. (See Appendices^ pp. 756
837, 1071, and 1074.)

Local Extension.—In Fig. 347 a test strip has been taken
12" long, and divisions marked across it at one inch apart, then
the actual extensions within each inch measured, and set up as
ordinates on the line A B ; c D E is the curVe shewing distribution
of extension, and is seen to increase very greatly towards the fifth